Protective role of the ELOVL2/docosahexaenoic acid axis in glucolipotoxicity-induced apoptosis in rodent beta cells and human islets
Dietary n-3 polyunsaturated fatty acids, especially docosahexaenoic acid (DHA), are known to influence glucose homeostasis. We recently showed that Elovl2 expression in beta cells, which regulates synthesis of endogenous DHA, was associated with glucose tolerance and played a key role in insulin secretion. The present study aimed to examine the role of the very long chain fatty acid elongase 2 (ELOVL2)/DHA axis on the adverse effects of palmitate with high glucose, a condition defined as glucolipotoxicity, on beta cells.
We detected ELOVL2 in INS-1 beta cells and mouse and human islets using quantitative PCR and western blotting. Downregulation and adenoviral overexpression of Elovl2 was carried out in beta cells. Ceramide and diacylglycerol levels were determined by radio-enzymatic assay and lipidomics. Apoptosis was quantified using caspase-3 assays and poly (ADP-ribose) polymerase cleavage. Palmitate oxidation and esterification were determined by [U-14C]palmitate labelling.
We found that glucolipotoxicity decreased ELOVL2 content in rodent and human beta cells. Downregulation of ELOVL2 drastically potentiated beta cell apoptosis induced by glucolipotoxicity, whereas adenoviral Elovl2 overexpression and supplementation with DHA partially inhibited glucolipotoxicity-induced cell death in rodent and human beta cells. Inhibition of beta cell apoptosis by the ELOVL2/DHA axis was associated with a decrease in ceramide accumulation. However, the ELOVL2/DHA axis was unable to directly alter ceramide synthesis or metabolism. By contrast, DHA increased palmitate oxidation but did not affect its esterification. Pharmacological inhibition of AMP-activated protein kinase and etomoxir, an inhibitor of carnitine palmitoyltransferase 1 (CPT1), the rate-limiting enzyme in fatty acid β-oxidation, attenuated the protective effect of the ELOVL2/DHA axis during glucolipotoxicity. Downregulation of CPT1 also counteracted the anti-apoptotic action of the ELOVL2/DHA axis. By contrast, a mutated active form of Cpt1 inhibited glucolipotoxicity-induced beta cell apoptosis when ELOVL2 was downregulated.
Our results identify ELOVL2 as a critical pro-survival enzyme for preventing beta cell death and dysfunction induced by glucolipotoxicity, notably by favouring palmitate oxidation in mitochondria through a CPT1-dependent mechanism.
KeywordsAMPK Apoptosis Ceramide DHA ELOVL2 Glucolipotoxicity Mitochondrial β-oxidation Pancreatic beta cells Type 2 diabetes
AMP-activated protein kinase
Carnitine palmitoyltransferase 1
Very long chain fatty acid elongase 2
G-protein coupled receptor 120
Monounsaturated fatty acid
Poly (ADP-ribose) polymerase
Polyunsaturated fatty acid
Small interfering RNA
The authors thank C. K. Ng, University College Dublin, Belfield, Ireland, for help with preparing the manuscript.
HLS and LB contributed to the study concept and design, and to the analysis and interpretation of the data. LB, MCa, CR, NK, JV, KM and JD contributed to the acquisition of the data on caspase activity and the western blot experiments. LB and NK contributed to the acquisition and interpretation of the data from quantitative PCR experiments. LB and IH contributed to generate Elovl2 adenoviruses and interpretation of data using them. VL and CP-B contributed to the acquisition and interpretation of the data on [U-14C]palmitate metabolism. MB contributed to the preparation of human islets and analysis of the data. MCh and AB-Z performed and analysed the lipidomic data. CM, CC-G, PM, MI and BT contributed to the analysis of the data. HLS, LB and CM wrote/edited the manuscript with contributions from CP-B, CC-G, MI, PM and BT. HLS is responsible for the integrity of the work as a whole. All authors revised the article and approved the final version.
This project was supported by grants from the Centre National de la Recherche Scientifique (CNRS), the Innovative Medicines Initiative Joint Undertaking under grant agreement no. 155005 (IMIDIA), and the Agence Nationale de la Recherche (ANR PRCI-15-CE14-0027-01 BetaDiamark). LB received a doctoral fellowship supported by IMIDIA-ENSO. MCa received a doctoral fellowship from the Cardiovasculaire – Obésité – Rein – Diabète – Domaine d’Intérêt Majeur (CORDDIM), Ile de France. JV was supported by a postdoctoral fellowship from Université Paris Diderot.
Duality of interest
All authors declare that they have no duality of interest associated with this manuscript. The study sponsor was not involved in the design of the study, interpretation of the data or writing of the report.